The study of excitation-contraction coupling in the heart would be greatly facilitated if a permeabilized cefl preparation were available in which the cytosol is accessible to added agents but in which the t-tubules remain intact, pinched off from the surface sarcolemma. We wish to use such a preparation to study the role of non-junctional ryanodine receptors in the release of Ca from the SR induced by t-tubular depolarization. I think I have developed such a preparation. It shows ouabain-sensitive Na uptake and thapsigargin-sensitive Ca uptake which is released by caffeine. These properties indicate an intact but inverted sarcolemmal space, and an intact SR, respectively. The ends of the permeabifized cells stain with trypan blue, which suggests that the outer sarcolemma is disrupted at the ends of the cell, but not in the middle. I wish to demonstrate that the t-tubules form an enclosed space in this preparation by trapping fluorescein in the t-tubules when the cells are permeabilized, and then visualizing the trapped dye by imaging the cells with confocal microscopy.